Testable membrane sensor with two full bridges

ABSTRACT

A sensor is proposed that has a measuring diaphragm ( 2 ), with at least one resistor measurement bridge ( 7, 8 ); a deflection of the measuring diaphragm ( 2 ) causes mistuning of the respective measurement bridge ( 7, 8 ), and the resultant change in the bridge diagonal voltage can be evaluated. The sensor ( 1 ) has one resistor measurement bridge ( 7, 8 ) on each half ( 3, 4 ) of the measuring diaphragm ( 2 ), and in each of the resistor measurement bridges ( 7, 8 ), two opposed bridge branches (R 1,  R 4 ) are changed in their resistance values (ΔR) by radial compressive offset, and the respectively other bridge branches (R 2,  R 3 ) are altered in their resistances by radial or tangential elongation.

BACKGROUND OF THE INVENTION

The invention relates to a sensor, especially a pressure sensor.

By way of example, pressure sensors are known in which thin-filmresistor measurement bridges for measuring absolute pressures orpressure changes, particularly in hydraulic systems, are disposed on ameasurement diaphragm. Motions of the measurement diaphragm frompressure fluctuations lead to changes of resistance, because ofcompressive offsets or elongations of what as a rule are meanderingresistor tracks, in the various thin-film resistors. The thin-filmresistors are connected in a known manner to form a Wheatstonemeasurement bridge; the association of the thin-film resistors with thebridge branches or the regions on the pressure sensor diaphragm isselected such that the opposed resistors each vary in the samedirection, and a bridge diagonal voltage can be measured as a sensorsignal.

In the most frequent instances of applications of pressure sensors, suchas in hydraulic brake systems in motor vehicles, an accurate outputsignal corresponding to the pressure of the brake hydraulics(measurement range approximately 250 bar) must be generatable highlyreliably and in addition in as fail-safe a way as possible. Especiallyin systems critical to safety in the area of brake systems, such as theanti-lock system or traction control system, sensors are required, theperfect function of which must also be monitorable continuously. Otherapplications include monitoring functions in pneumatic systems and ininjection systems for delivering fuel in motor vehicles.

It is also known that the monitoring of pressure sensors that haveresistor measurement bridges is done in such a way that at specifiedtime intervals an absolute measurement of the individual resistors isdone, in order to detect changes, caused for instance by aging ordestruction (for instance from corrosion or breakage) of the resistorproperties of the individual thin-film resistors. Plastic deformationsof the pressure measuring diaphragm from overpressure or tearing of themost severely strained point in the middle of the diaphragm causeincorrect measurements. A change in resistors that vary in the samedirection in the bridge branches cannot be detected, without the specialprovisions already mentioned, since these changes compensate for oneanother by offset in the measurement bridge, and thus while themeasurement bridge appears unchanged from outside, nevertheless itssensitivity changes and thus mistakes.

The resistors of the measurement bridge that each change in the samedirection are located on the pressure measuring diaphragm preferentiallyat locations having the same mechanical properties with respect totensile elongation or compressive offset (either in the middle or on theedge of the pressure measuring diaphragm) and are therefore under equalstrain; their deviations behave accordingly. Plastic deformations of thepressure measuring diaphragm thus also exhibit the same undetectablesignal errors. Another known possibility of detecting such errors is torepeat the comparison of the individual resistors with a stablereference resistor at certain intervals. The reference resistor, whichis stable over its entire life, can be connected parallel to a bridgeresistor for this purpose and therefore used for monitoring changes inthe bridge output signal.

The known sensors with the special monitoring mechanisms discussed havethe disadvantage above all that a constant switchover is necessarybetween the test/monitoring mode and pressure sensing, which greatlyreduces the dynamics of the sensor since the reference measurement takestime. Moreover, demands for failsafeness and redundancy cannot be met bythese provisions.

SUMMARY OF THE INVENTION

Accordingly, it is an object of present invention to provides a sensorwhich avoids the disadvantages of the prior art.

In keeping with these objects, one feature of present invention residesin a sensor in which in a first resistor measurement bridge all fourbridge branches are acted upon by radially acting mechanical tensions onthe measuring diaphragm and these bridge branches stretched by radialelongation being located in the center of the measuring diaphragm, andin the second resistor measurement bridge unlike the first, the bridgebranches that are acted upon by a tangential elongation are located in aperipheral region of the measuring diaphragm and are oriented such thattangential mechanical tensions act upon them.

The sensor according to the invention is advantageous, in particularbecause as a result of disposition of two mutually independent resistormeasurement bridges each on one half of the diaphragm, monitoring of thefunctionality of the sensor can be done during operation without specialreference measurements. The availability of the sensor is increased aswell, since even if one resistor measurement bridge fails, emergencyoperation of the system with the other measurement bridge is assured.

In a preferred embodiment of the sensor of the invention, the thin-filmresistors of at least one of the two resistor measurement bridges arearranged on the measuring diaphragm in such a way that radialelongations/compressiive offsets of the measuring diaphragm cause anincrease or decrease in resistance. For the other resistor measurementbridge in each case, the thin-film resistors opposite one another in thebridge are arranged on the measuring diaphragm in such a way that atangential extension is preferentially detected in the peripheral regionof the diaphragm and causes an increase in resistance. By utilizing thetangential effect, these resistors are less severely strained, and thedeviation over their lifetime is thus also lower.

Since the two measurement bridges of the embodiment described abovebehave differently with respect to the bridge diagonal signal over theirlifetime, since the thin-film resistors for detecting the tangentialelongation detect a different diaphragm motion from the radialelongation or compressive offset, a simple functional monitoring can bedone by comparing the two bridge signals. Plastic deformations of thepressure measuring diaphragm can also be detected unequivocally in thebridge offset in this way, since the two bridge diagonal signals driftmarkedly apart as a result. Aging phenomena and mechanical orphysical-chemical effects influence the sensitivity of the two bridgesdifferently, so that the sensitivity can be detected by means of acomparison.

BRIEF DESCRIPTION OF THE DRAWINGS

One exemplary embodiment of the sensor of the invention will beexplained in conjunction with the drawing. Shown are:

FIG. 1, a plan view on a measuring diaphragm of a pressure sensor;

FIG. 2, an electrical circuit diagram for the right-hand measurementbridge;

FIG. 3, an electrical circuit diagram for the left-hand measurementbridge;

FIG. 4, a detail view of a thin-film resistor; and

FIG. 5, a graph showing the mechanical tension/elongations on themeasuring diaphragm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, a sensor 1 is shown that serves as a pressure sensor fordetecting pressure conditions in the brake hydraulics of a motorvehicle. The sensor 1 includes a measuring diaphragm 2 (for instance ofmetal), on which thin-film resistors R1, R2, R3 and R4 (for instance ofpolycrystalline silicon) are applied to each of two sensor halves 3 and4. The thin-film resistors R1-R4 are contacted on the measuringdiaphragm 2 at points 5, and for purposes of external connection, thepoints 5 are each extended to contact pads 6. For the sake ofsimplicity, this is shown in FIG. 1 only for the resistor R1 on theright-hand sensor half 4, as an example.

FIG. 2 and FIG. 3 show the electrical substitute circuit diagrams forthe resistors R1-R4 on sensor half 3 (FIG. 2) and sensor half 4 (FIG. 3)each of which forms a respective Wheatstone measurement bridge 7 and 8.For evaluation as a sensor output signal, the bridge diagonal voltagesUml (FIG. 2) and Umr (FIG. 3) are available.

One exemplary embodiment of one of the thin-film resistors R1-R4 isshown in FIG. 4, in which the meandering structure of resistor tracks 9between the points 5 can be seen. The resistors R1-R4 here undergo achange in their resistance (+ΔR) on an elongation (+Δ1) in the directionindicated. In the case of resistors made of other materials, ageometrical structure may be chosen in order to attain the samemeasurement effect.

In FIG. 5, a graph of the courses of the mechanical tensions σ and theresultant elongations or compressive offsets ε in radially differentregions of the measuring diaphragm 2 is shown. An explanation of thisgraph will be made in conjunction with the description of the exemplaryembodiment, in particular with reference to FIGS. 1-3.

The bridge resistors R1 and R4 of the right sensor half 4 (see FIGS. 1and 3) are located in the peripheral region of the measuring diaphragm 2in the vicinity of the mechanical fastening, and the bridge resistors R2and R3 are located in the center of the measuring diaphragm 2. Upon adeflection or bulging of the measuring diaphragm 2 from an increase inpressure, the bridge resistors R2 and R3 in the center of the measuringdiaphragm 2 undergo an elongation in the same direction as a result ofthe radially acting mechanical tension, which leads to an increase intheir resistances (+ΔR). In the case of the bridge resistors R1 and R4in the peripheral region, a compressive offset occurs from the bulgingin the same direction in the fastening region of the measuring diaphragm2. Again because of the radially acting mechanical strain, this causes areduction in the resistances (−ΔR) of the bridge resistors R1 and R4.The resultant mistuning of the measurement bridge 8 can be evaluated viathe altered bridge diagonal voltage Umr.

In the left sensor half 3, the bridge resistors R1 and R4 (see FIGS. 1and 2) are arranged identically to the corresponding bridge resistors R1and R4 in the right sensor half 4 and therefore undergo the sameresistance changes as well. To achieve the advantageous propertiesdiscussed in the introduction to the specification, however, the bridgeresistors R2 and R3 of the measurement bridge 7 are likewise disposed inthe peripheral region of the pressure measuring diaphragm 2,specifically in such a way that a tangential elongation effect of thediaphragm surface caused by the mechanical tension is evaluated. Themeandering resistor tracks 9 of the resistors R2 and R3 do undergo anincrease in resistance (+ΔR) from elongation here, but the mechanicalinteractions between a pressure change (+Δp) and the mistuning of thebridge diagonal voltage Uml are different from the interactions on theright sensor half 4.

Because of the different evaluations of a pressure change (Δp) in thetwo sensor halves 3 and 4, many errors in the sensor 1 (for instancefrom aging, corrosion or membrane breakage), that otherwise causechanges in the same direction in the bridge resistors and are thuscompensated for in the bridge offset, can thus be detected. The bridgeresistors R2 and R3 of the left measurement bridge 7 are also located ina mechanically relatively little stressed region of the measuringdiaphragm 2, so that the reliability of the left-hand measurement bridge7 is very high, and thus the emergency operation properties of thesensor 1 are improved.

The graph of FIG. 5 schematically shows some typical courses of themechanical tension σ over the radius r of the measuring diaphragm 2 andthe resultant elongations/compressive offsets ε at the bridge resistorsR1-R4. The course of the radially acting tension σr is shown in curve10, and the course of the tangentially acting tension σt is shown incurve 11. Curve 12 shows the course of the radial elongation εr, andcurve 13 shows the course of the tangential elongation εt with respectto the right-hand vertical coordinate axis.

From curve 12 in FIG. 5, the transition from the pronounced elongationat the center of the measuring diaphragm 2 (r=0) to the compressiveoffset in the peripheral region, caused by the radial tension εr (curve10) which results from a pressure increase Δp, can clearly be seen. Thetangential tension σt (curve 11) and the resultant elongation εt,conversely, have a markedly flatter course and therefore have adifferent dependency on the pressure change Δp. In order nevertheless toattain merely equal measurement ranges in the normal evaluation of thebridge diagonal voltages, the bridge resistors R2 and R3 of the leftsensor half 3 can be shifted to a region of the measuring diaphragm 2 inwhich a comparable elongation to the bridge resistors R2 and R3 of theright sensor half 4 is detected. The most favorable possibilities forlocating the bridge branches are represented in the graph of FIG. 5 bysmall circles, which are located approximately symmetrically (+ε1; −ε1)to the zero point of the elongation-compressive offset axis ε.

What is claimed is:
 1. A sensor, comprising a measuring diaphragm; and aresistor measurement bridge means arranged on said measuring diaphragmso that a deflection of said measuring diaphragm causes mis tuning ofsaid measuring bridge means and a resulting change in a bridge diagonalvoltage is evaluatable, said resistor measurement bridge means includinga first resistor measurement bridge and a second resistor measurementbridge each arranged on each half of said measuring diaphragm, saidfirst resistor measurement bridge having all four bridge branches whichare acted upon by radially acting mechanical tensions on said measuringdiaphragm and said bridge branches stressed by radial elongation beinglocated in a center of said measuring diaphragm, said second resistormeasurement bridge unlike said first resistor measurement bridge havingbridge branches that are acted upon by a tangential elongation andlocated in a peripheral region of said measuring diaphragm and orientedsuch that tangential mechanical tensions act upon them.
 2. A sensor asdefined in claim 1, wherein said bridge branches have resistors whichare formed as thin-film resistors with meandering resistor tracks whichduring an extension between connecting points undergo a resistanceincrease and during a compression undergo a resistance decrease.
 3. Asensor as defined in claim 2, wherein said thin-film resistors arecomposed of polycrystalyne silicone.
 4. A sensor as defined in claim 2,wherein said bridge branches of said second resistor measurement bridgeupon which tangential elongation acts and said bridge branches of saidfirst resistor measurement bridge upon which radially acting mechanicaltensions act are arranged in a region between a center and an edge ofsaid measuring diaphragm, so that during disturbance free operation ofthe sensor a relative sensitivity with regard to mechanical tensions onsaid measuring diaphragm is provided.
 5. A sensor as defined in claim 1,wherein said sensor is formed as a pressure sensor for monitoringpressure conditions in hydraulic and/or pneumatic systems in a motorvehicle.